By way of example, lithography installations are used in the production of integrated circuits or ICs for imaging a mask pattern in a mask onto a substrate such as e.g. a silicon wafer. In so doing, a light beam generated by an optical system is directed through the mask onto the substrate.
Currently, EUV lithography installations which use light with a wavelength in the range from 5 nm to 30 nm, in particular 13.5 nm, are in development. “EUV” denotes “extreme ultraviolet”. In the case of such lithography installations, because of the high absorption of light of this wavelength by most materials, reflective optics, that is to say mirrors, have to be used instead of—as previously—refractive optics, that is to say lenses. The provision of mirrors in the form of so-called mirror fields (also referred to as mirror arrays), which may include several hundred thousand mirrors, is known. Here, the mirrors are each tiltable about one axis or two mutually perpendicularly oriented axes in order to guide the light on a suitable path to the substrate to be exposed. For the purposes of tilting the mirrors, actuators which are actuated by way of a control loop are assigned to the former. An apparatus for monitoring the tilt angle of a respective mirror is provided as part of the control loop.
By way of example, WO 2010/094658 A1 has disclosed such an apparatus for monitoring at least one mirror. The apparatus includes an acquisition device and a pattern source. A pattern provided by the pattern source is mirrored by the mirror onto the acquisition device. The tilt angle of the mirror is determined in a manner dependent on the light acquired by the acquisition device.
The mirrors of a mirror field in particular regularly have a resonant frequency of several hundred hertz, and so a quick measurement of the tilt angle is desirable. As a result of this, faster regulation of the tilt angle could be made possible, as a result of which, ultimately, the quality of the mask pattern imaged on the substrate can be improved. Moreover, the throughput of the lithography installation can be increased.